The present invention relates to ceramic articles and, more particularly, to porous ceramic articles or bodies which possess on one or more surfaces a thin porous ceramic membrane.
It is known in the art to produce porous ceramic articles by a variety of means resulting in structures having a predominant open-cell porosity, particularly useful as filtering elements, or a predominant closedcell porosity useful in applications where strong low-density structures of low thermal conductivity or buoyancy or the like are advantageous.
For example, the use of porous ceramics as filtering elements has become a matter of recent interest in response to increasingly stringent governmental regulations in the United States and European countries regarding maximum allowable particulates (e.g., soot) in the exhaust of automobile or truck diesel engines. Porous ceramic elements have been proposed as filters through which diesel engine exhaust can be passed in order to trap particulates and produce a low-particulate exhaust, and wherein the particulates can then periodically be burned-out (i.e., combusted) within the element in order to regenerate it for continued use. Ceramic materials theoretically lend themselves well to such use because of their excellent structural and dimensional stability under the high temperature conditions encountered during exhaust filtration and periodic regeneration.
Most notable in these efforts has been the utilization of so-called ceramic monolithic honeycomb filtering elements as described, for example in U.S. Pat. Nos. 4,276,071 and 4,364,761 assigned to General Motors Corp.; U.S. Pat. Nos. 4,329,162; 4,415,344; 4,416,675; 4,416,676; 4,417,908; 4,419,108 and 4,420,316 assigned to Corning Glass; and U.S. Pat. Nos. 4,283,210; 4,293,357; 4,307,198; 4,340,403; and 4,364,760 assigned to NGK Insulators. Essentially, these elements consist of a monolithic ceramic having a multitude of internal parallel chambers separated by thin porous ceramic internal walls, with a number of the chambers being end-sealed so as to force particulate-containing exhaust gas to traverse across a porous wall before exiting the element. Generally, these elements are formed by an extrusion process, and materials are included in the composition which are burned out during the firing process to provide the requisite porosity in the internal filtering surfaces. In another process, as reflected in U.S. Pat. No. 4,363,644 assigned to Nippon Soken, foamed, structural polyurethane systems are utilized in admixture with inorganic materials in processes wherein the polyurethane burns out during firing so as to leave behind a ceramic structure having a variety of interconnected open cells for trapping particulates.
The efficiency of porous ceramic filters for removal of particulates from diesel exhaust generally involves a compromise between use of sufficiently small pores to efficiently trap particulates and use of sufficiently large pores to insure a relatively easy path for the exhaust without creation of significant back pressure. To date, the art has not adequately solved this dilemma.
Additionally, a number of potential applications for low-density ceramics exist which require or can be enhanced, functionally or aesthetically, by provision of smoother surfaces than heretofore technically and/or economically possible using known techniques for forming porous ceramic articles.